ABSTRACT

The literature reveals difficulties in taxonomic and phylogenetic delimitations of Miconia Ruiz & Pav. and Leandra Raddi (Melastomataceae), it’s suggested that leaf anatomical characters, particularly the trichomes, can be useful within the genera. Characters related to the trichomes and leaf blade structure of 15 species of Miconieae (five species of Leandra and ten species of Miconia) were evaluated as a taxonomic criterion to separate the selected species. In addition, some considerations were made about the structure of the leaves, which can be affected by environmental factors. Leaf samples were obtained from herbaria and anatomically investigated using optical microscopy and scanning electron microscopy. Leaves are hypostomatous, dorsiventral or isobilateral, and exhibit epidermis in general with non- glandular and glandular trichomes. A hypodermis has been found only in Miconia theaezans. Conical trichomes and mixed trichomes (non-glandular and glandular) are present only in the five species of Leandra. Dendritic trichomes have been found in species of both genera. Leaf features of potential taxonomic significance are the trichome types, presence of hypodermis, number of layer palisade parenchyma, occurrence of gelatinous fibers, midrib structure, and margin structure. Xeromorphic and sclerophyllous features are present in Leandra and Miconia species, and Leandra microphylla may be considered a xerophyte.

Keywords:
Conical trichome; dendritic trichome; hypodermis; leaf margin; mesophyll structure; midrib structure.

Introduction

Miconieae Mart. is considered the tribe with the greatest diversity of species within Melastomataceae (Michelangeli et al. 2008Michelangeli FA, Judd WS, Penneys DS et al. 2008. Multiple events of dispersal and radiation of the tribe Miconieae (Melastomataceae) in the Caribbean. The Botanical Review 74: 53-77. ; Penneys et al. 2022Penneys DS, Almeda F, Reginato M, Michelangeli FA, Goldenberg R, Fritsch PW, Stone D. 2022. A New Melastomataceae classification informed by molecular phylogenetics and morphology. In: Goldenberg R, Michelangeli FA, Almeda F (eds.) Systematics, evolution, and ecology of Melastomataceae. Cham, Springer. p. 109-165.; Ulloa Ulloa et al. 2022Ulloa Ulloa C, Almeda F, Goldenberg R et al. 2022. Melastomataceae: Global diversity, distribution, and endemism. In: Goldenberg R , Michelangeli FA, Almeda F (eds.). Systematics, Evolution, and Ecology of Melastomataceae. Cham, Springer , 2022. p. 3-28.). Among its genera, Miconia Ruiz & Pav. and Leandra Raddi deserve to be highlighted because they are genera considered the most representative in number of species of the tribe (Michelangeli et al. 2004Michelangeli FA, Penneys DS, Giza J, Soltis D, Hils MH, Skean JD. 2004. A preliminary phylogeny of the tribe Miconieae (Melastomataceae) based on nrITS sequence data and its implications on inflorescence position. Taxon 53: 279-290.; Oliveira 2007Oliveira JBS. 2007. Anatomia foliar como subsídio à taxonomia de Miconia Ruiz et Pav. (Melastomataceae) em Pernambuco-Brasil. MSc Thesis, Universidade Federal Rural de Pernambuco, Brazil.; Camargo 2008Camargo EA. 2008. O gênero Leandra, seções Carassanae, Niangae e Secundiflorae (Melastomataceae) no Paraná. MSc Thesis, Universidade Federal do Paraná, Brazil.; Penneys et al. 2022Penneys DS, Almeda F, Reginato M, Michelangeli FA, Goldenberg R, Fritsch PW, Stone D. 2022. A New Melastomataceae classification informed by molecular phylogenetics and morphology. In: Goldenberg R, Michelangeli FA, Almeda F (eds.) Systematics, evolution, and ecology of Melastomataceae. Cham, Springer. p. 109-165.).

In Brazil, Miconia and Leandra are widely distributed in areas of Atlantic Rainforest and “Cerrados”, and are considered typical elements of these vegetation formations (Flora e Funga do Brasil 2023Flora e Funga do Brasil. 2023. http://floradobrasil.jbrj.gov.br/. 06 Dec. 2023.
http://floradobrasil.jbrj.gov.br/... ). Currently, 176 species of Leandra and 271 species of Miconia s.s. have been recorded in Brazil, with the genera comprising ca. 200 and 2000 species, respectively (Michelangeli et al. 2019Michelangeli FA, Goldenberg R, Almeda F et al. 2019. Nomenclatural novelties in Miconia (Melastomataceae: Miconieae). Brittonia 71: 82-121. ; Goldenberg et al. 2023Goldenberg R, Bacci LF, Caddah MK, Meirelles J. Miconia. In: Flora e Funga do Brasil. Rio de Janeiro, Jardim Botânico do Rio de Janeiro. https://floradobrasil.jbrj.gov.br/FB9666/. 06 Dec. 2023.
https://floradobrasil.jbrj.gov.br/FB9666... ).

Phylogenetic analyses of Miconieae at the genus and tribe levels have shown a lack of monophyly among traditional taxa, and that the morphological characters used in the tribe's taxonomy are tenuous and not very consistent (sensuPenneys et al. 2022Penneys DS, Almeda F, Reginato M, Michelangeli FA, Goldenberg R, Fritsch PW, Stone D. 2022. A New Melastomataceae classification informed by molecular phylogenetics and morphology. In: Goldenberg R, Michelangeli FA, Almeda F (eds.) Systematics, evolution, and ecology of Melastomataceae. Cham, Springer. p. 109-165.). Leandra and Miconia were treated here as distinct genera in Miconieae, but recently Michelangeli et al. (2022Michelangeli FA, Nicolas AN, Ocampo G et al. 2022. Why recognize Miconia as the only genus in tribe Miconieae (Melastomataceae)? In: Goldenberg R, Michelangeli FA, Almeda F (eds.) Systematics, evolution, and ecology of Melastomataceae. Cham, Springer. p. 235-254.) proposed that the tribe be recognized with the single genus Miconia. With this monogeneric proposal, these authors consider it urgent to have a consistent revised infrageneric taxonomy, within a framework that encompasses the entire genus.

Leaf structure and trichome morphology of leaves or flowers have been used as significant characters in the taxonomy of Melastomataceae (Metcalfe & Chalk 1957Metcalfe CR, Chalk L. 1957. Anatomy of the dicotyledons - leaves, stem, and wood in relation to taxonomy with notes on economic uses. Oxford, Clarendon Press.; Wurdack 1986Wurdack JJ. 1986. Atlas of hairs for Neotropical Melastomataceae. Smithsonian Contributions to Botany 63: 1-80.; Guimarães et al. 1999Guimarães PJF, Ranga NT, Martins AB. 1999.Morfologia dos tricomas em Tibouchina sect. Pleroma (D. Don) Cogn. (Melastomataceae). Brazilian Archives of Biology and Technology 42: 485-493.; Reis et al. 2005Reis CD, Bieras AC, Sajo MDG. 2005. Anatomia foliar de Melastomataceae do Cerrado do estado de São Paulo. Brazilian Journal of Botany 28: 451-466. ; Oliveira 2007Oliveira JBS. 2007. Anatomia foliar como subsídio à taxonomia de Miconia Ruiz et Pav. (Melastomataceae) em Pernambuco-Brasil. MSc Thesis, Universidade Federal Rural de Pernambuco, Brazil.; Donato et al. 2018Donato AM, Silva FB, Santos Rios E, Almeida JR. 2018. Uma abordagem sobre a estrutura anatômica de Chaetostomaglaziovii Cogn. - Melastomataceae - e seu estabelecimento bem sucedido em campos de altitude. Revista Internacional de Ciências 8: 115-128.; Carmo et al. 2019Carmo A, Pacifico R, Gonçales-Silva R, Sbais PG, Fidanza K, Souza LA. 2019. Epidermal micromorphology and venation patterns of Microlicieae (Melastomataceae) leaves: Looking for new characters for the taxonomy of this neotropical tribe. Flora 261: 151-494.; Gonçales-Silva et al. 2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.; Gonçales et al. 2023Gonçales R, Carmo AAO, Santos AF, Souza LA. 2023. Floral indumentum of Miconia: In search of diagnostic characters for the tribe Miconieae. Revista Mexicana de Biodiversidad 94: e945272.). Leaf structural characters investigated as a contribution to the taxonomy of Leandra or Miconia were reported by Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.) and Müller et al. (2020Müller AO, Franco AA, Ribeiro Júnior NG, Gressler E, Rocha VLP, Silva IVD. 2020. Estratégias adaptativas foliares de Miconia nervosa (Melastomataceae) na Amazônia Matogrossense. Rodriguésia 71: e01052018.). In Leandra and Miconia the trichome morphology provide many important features which are of taxonomic value at the specific level (Gonçales-Silva et al. 2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.; Gonçales et al. 2023Gonçales R, Carmo AAO, Santos AF, Souza LA. 2023. Floral indumentum of Miconia: In search of diagnostic characters for the tribe Miconieae. Revista Mexicana de Biodiversidad 94: e945272.).

The species of Leandra and Miconia selected for study include plants that live in forests and rupestrian fields, five species of Leandra (or 2.27% of 176 spp.) and 10 of Miconia (4% of 270 spp.). The main purpose of this investigation was to ascertain the usefulness of leaf structure and trichome morphology as criterion for characterization of species of both genera. In addition to ensuring data for use in Miconieae taxonomy, we consider it important to make some leaf structural ecological considerations, especially because these species occur in different environments.

Material and Methods

Plant material and sample preparation

Three samples of leaves were used for each of the 15 species studied of Miconieae belonging to the genera Leandra and Miconia (Tab. 1) were analyzed anatomically. Completely expanded leaves from the third to fifth stem nodes were obtained from vouchers from the herbarium of the Municipal Botanical Museum of Curitiba (MBM) and from the State University of Maringá (HUEM), Brazil. The leaves were previously rehydrated in boiling water, immersed in 5% potassium hydroxide (KOH) for two hours and submitted to the 10%, 30%, 50% and 70% ethyl series, according to Smith & Smith (1942Smith FH, Smith EC. 1942. Anatomy of the inferior ovary of Darbya. American Journal of Botany 29: 464-471.). Successively, the leaves were fixed in a solution of Glutaraldehyde in phosphate buffer (Karnovsky 1965Karnovsky JM. 1965. A formaldehyde glutaraldehyde fixative of high osmolality for use in electron microscopy. Journal of Cell Biology 27: 1-149.) and in FAA 50 (Formaldehyde, Acetic acid and 50% Ethyl alcohol; 1:1:18) and stored in 70% ethanol (Johansen 1940Johansen DA. 1940. Plant microtechnique. New York, McGraw-Hill.).

Analysis under a light microscope

Median and margin fragments of the leaf lamina with 1×1 size were submitted to 80%, 90% and 100% ethyl alcohol series and embedded in Leica^® HistoResin, according to the manufacturer's instructions. The embedded material was sectioned transversely to the rotation microtome (Easy Path and HistoCore BIOCUT), in sections from 5 to 7μm in thickness, and stained in 0.05% Toluidine Blue in acetate buffer (pH 4.7) (O’Brien et al. 1964O’Brien TP, Feder N, Mccully ME. 1964. Polychromatic staining of plant cell walls by toluidine blue O. Protoplasma 59: 368-373.). The sections were illustrated using a Leica ICC50 light microscope with an attached digital camera, using the Leica Application Suite 1.8 software, through digital image capture.

Analysis under a scanning electron microscope (SEM)

The scanning electron microscope (SEM) analysis was performed with glutaraldehyde 5% fixed material. After washing in 0.1 M sodium cacodylate buffer, leaf fragments were dehydrated in graded ethanol series, critical point drying with CO₂, mounted on aluminum stubs, coated with gold (Horridge & Tamm 1969Horridge GA, Tamm SL. 1969. Critical point drying for scanning electron microscopic study of ciliary motion. Science 163: 817-818. ). The samples were examined in a Scanning Electron Microscope Quanta 250 (Fei-Oxford Instruments, Oxfordshire, United Kingdom) operating between 15-25kV, and the images were captured digitally. Lamina trichomes were described according to Wurdack (1986Wurdack JJ. 1986. Atlas of hairs for Neotropical Melastomataceae. Smithsonian Contributions to Botany 63: 1-80.)’s atlas of hairs (trichomes). The convex walls of the glabrous leaf epidermis of Miconia dodecandra were described according to the terminology of Barthlott & Hunt (2000Barthlott W, Hunt D. 2000. Seed-diversity in the Cactaceae, subfamily Cactoideae. Succulent Plant Research 5. Authors edition.).

Results and Discussion

Leaf features and their taxonomic importance in Miconia and Leandra

The surface of the leaf lamina is flat, but this surface is wavy (undulations) in Leandra microphylla (Fig. 1 A ) and Leandra nianga (Fig. 1 B ) (Tab. 2). All studied species of Leandra and Miconia have hypostomatous lamina (Fig.1 C, G, I), and consists of uniseriate and cuticularized epidermis on both surfaces (Fig.1A-I). In general, the cuticular layer is thick on the adaxial surface and comparatively thinner on the abaxial surface (Fig.1A-I). Seen in cross-section the epidermal cells of both surfaces are rounded, rectangular, square or slightly prismatic; it is common for cells on the abaxial surface to be slightly smaller in size than those on the adaxial surface (Fig. 1 A -I). Phenolic derivatives were seen in the epidermal cells of the adaxial surface of M. dodecandra (Fig. 2 G ), Miconia prasina (Fig. 1 D ) and Miconia valtherii (Fig. 1 C ) and on the abaxial surface of M. prasina (Fig. 1 D ). Trichomes are always present, but never in Miconia pusiliiflora.

Figure 1.
Lamina structure of L. microphylla (A), L. nianga (B), M. valtherii (C), M. prasina (D), M. theaezans (E), M. budlejoides (F), M. sellowiana (G), M. pusilliflora (H) and M. splendens (I), in cross-sections. A,B. Laminae with re-entrances. C,D. Dorsiventral laminae with 2 cell-layers of palisade parenchyma, and phenolic derivatives in the epidermis and mesophyll. E. Dorsiventral lamina with hypodermis and bundle sheath extension (white arrow). F. Dorsiventral lamina (1 cell-layer of palisade parenchyma) exhibiting dendritic and glandular trichomes. G. Isobilateral lamina with druses in the mesophyll. H,I. Dorsiventral laminae (2-3 cell-layers of palisade parenchyma) exhibiting subepidermal druses and phenolic derivatives. (ae=adaxial surface epidermis; dt=dendritic trichome; hy=hypodermis; pp=palisade parenchyma; sp=spongy parenchyma; st=stomata; white arrow=druse). Scale bars: 150µm (A,B), 50µm (C-I).

Figure 2.
Midrib and margin structure of Leandra microphylla (A), L. cinerascens (B), Miconia theaezans (C), L. purpurascens (D), L. nianga (E), L. purpurascens (F) and M. dodecandra(G), in cross-sections. A-D. Midribs with almost closed (A) and U-shaped vascular cylinders; presence of mesophyll in A, collenchyma in B and C, and collenchyma and mesophyll in D (adaxial surface). E-G. Margins with gelatinous fibers, collenchyma and palisade parenchyma, respectively. (ab=additional vascular bundle; co=collenchyma; gf=gelatinous fibers; me=mesophyll; pp=palisade parenchyma). Scale bars: 300µm (A,B), 200µm (C-E), 100µm (G), 50µm (F).

A subepidermal layer was found on the adaxial surface of the leaf of Miconia theaezans (Fig. 1 E ), which was interpreted by Costa (1977Costa CG. 1977. Miconia theaezans (Bomp.) Cogn. (Melastomataceae): Considerações anatômicas. Rodriguésia 29: 7-92.) as hypodermis based on the observation that it originates from the subepidermal layer of the young mesophyll that undergoes periclinal divisions. This layer is made up of achlorophyllous, thin-walled parenchymatous cells, some of which are larger and have druses. In the study of leaves of some species of Leandra and Miconia, carried out by Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.), a hypodermis was also recorded, but on the abaxial face of Miconia ligustroides (DC.) Naudin. Oliveira (2007Oliveira JBS. 2007. Anatomia foliar como subsídio à taxonomia de Miconia Ruiz et Pav. (Melastomataceae) em Pernambuco-Brasil. MSc Thesis, Universidade Federal Rural de Pernambuco, Brazil.) also refers to the presence of hypodermis in a leaf of Miconia latecrenata (DC.) Naudin. Hypodermis is referred to in the anatomical characterization of Melastomataceae for the genus Miconia by Metcalfe & Chalk (1957Metcalfe CR, Chalk L. 1957. Anatomy of the dicotyledons - leaves, stem, and wood in relation to taxonomy with notes on economic uses. Oxford, Clarendon Press.).

The Miconieae leaves investigated here are dorsiventral or bifacial (Fig. 1 A -I), but Miconia sellowiana exhibits isobilateral mesophyll (Fig. 1 G ) (Tab. 2). The number of cell-layers of the palisade parenchyma varies between species (Tab. 2) and the spongy parenchyma is multiseriate (Fig. 1 AI ). In the mesophyll, cells with druses and cells with phenolic derivatives frequently occur. Some species, such as M. pusilliflora (Fig. 1 H ), M. sellowiana (Fig. 1 G ) and M. splendens (Fig. 1 I ), show subepidermal idioblasts with druse within the palisade parenchyma. It is noteworthy that the palisade layer may exhibit phenolic content in the subepidermal palisade layer of M. valtherii (Fig. 1 C ) and M. prasina (Fig. 1 D ).

Dorsiventral or centric leaf is recorded as an anatomical character for Melastomataceae (Metcalfe & Chalk 1957Metcalfe CR, Chalk L. 1957. Anatomy of the dicotyledons - leaves, stem, and wood in relation to taxonomy with notes on economic uses. Oxford, Clarendon Press.), and for some other studied species of Leandra and Miconia (Oliveira 2007Oliveira JBS. 2007. Anatomia foliar como subsídio à taxonomia de Miconia Ruiz et Pav. (Melastomataceae) em Pernambuco-Brasil. MSc Thesis, Universidade Federal Rural de Pernambuco, Brazil.; Gonçales-Silva et al. 2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.). Isobilateral leaf, like that found here in M. sellowiana, seems to be infrequent in the family. Attention has been drawn to the fact that this species typically occurs in the rupestrian fields of Guartelá State Park (Tibagi, state of Paraná, Brazil), which are characterized by small-sized vegetation, which develops in shallow soil or rocky outcrops, and has structural adaptations aimed at water stress. It should be noted that Reis et al. (2005Reis CD, Bieras AC, Sajo MDG. 2005. Anatomia foliar de Melastomataceae do Cerrado do estado de São Paulo. Brazilian Journal of Botany 28: 451-466. ) also investigated the leaf of M. sellowiana occurring in the Cerrado of the state of São Paulo (Brazil), but interpreted the palisade parenchyma of the abaxial surface of the mesophyll, with shorter cells, as belonging to the spongy parenchyma.

The smaller veins consist of collateral vascular bundle and are surrounded by parenchymatic bundle sheath (endoderm) (Fig. 1 G -I). Parenchymatous or collenchymatous bundle sheath extensions were recorded on the laminas of Miconia cinerascens and M. theaezans (Fig. 1 E ), on both sides of the vascular bundle. All species have a midrib (Fig. 2 A -D) and medium-sized veins with a protrusion on the abaxial surface. The midrib consists of uniseriate epidermis on the abaxial surface, with a generally thick cuticular layer, and trichomes may occur (Fig. 2 A -D). Under this epidermis there are collenchyma and parenchyma (Fig. 2 A -D), which may have cells with druses or phenolic derivatives. There is a U or V vascular cylinder, with phloem on both sides of the xylem, cambium and reduced secondary vascular growth (Fig. 2 A -D); in addition to this larger cylinder, one (Fig. 2 C ) or two smaller additional bundles may occur on the adaxial region of the rib (Tab. 2). The adaxial region of the vein may contain mesophyll, with palisade or spongy parenchyma or be devoid of it, showing collenchyma or parenchyma (Fig. 2 A -D) (Tab. 2). The midrib of Leandra purpurascens has collenchyma and mesophyll on the adaxial region (Fig. 2 D ). The epidermis of the adaxial surface of the midrib shows much resemblance to the epidermis of the lamina (Fig. 2 A -D).

The vascular system of the midrib of almost all 15 species of Leandra and Miconia is characterized by the form of an open arch, but in the case of L. microphylla (Fig. 2 A ) this arch is almost closed. As pointed by Reis et al. (2005Reis CD, Bieras AC, Sajo MDG. 2005. Anatomia foliar de Melastomataceae do Cerrado do estado de São Paulo. Brazilian Journal of Botany 28: 451-466. ) most species of Miconia and two species of Leandra [Leandra aurea (Cham.) Cogn. and Leandra lacunosa Cogn.] have a closed arch. Therefore, this feature may be taxonomically useful for separating species within the genus, but it is unreliable for separating both genera.

The margin of the leaf lamina has epidermis, eventually with trichomes, and the nature of the subepidermal tissue may vary (Tab. 2). The margin of L. purpurascens has collenchyma and chlorophyll tissue (palisade and spongy parenchyma) (Fig. 2 F ), while M. dodecandra has only palisade parenchyma (Fig. 2 G ). Leandra nianga's margin deserves some attention because it exhibits gelatinous fibers (Fig, 2E).

The leaves of Leandra and Miconia possess many anatomical features of potential taxonomic significance (Tab. 2), particularly associated with the types of trichomes (Tab. 3). For Leandra, features such as number of palisade parenchyma cell-layers, presence of gelatinous fibers, arrangement of the vascular system of the midrib (open or nearly closed arch), mesophyll in the midrib, presence of a smaller vascular bundle (additional bundles) on the adaxial region of the midrib and margin lamina structure can be employed. In the case of Miconia species, the significant characters are the presence of hypodermis, presence of crystal in subepidermal cells, type of mesophyll (dorsiventral or isobilateral), number of palisade parenchyma layers, additional vascular bundles on the adaxial region of the midrib, mesophyll on the adaxial region of the midrib and leaf margin structure.

Trichome types as a diagnostic character in the genera

Trichomes are present in 14 of the 15 species studied, on both surfaces or on a single epidermal surface; only M. pusilliflora does not have trichomes on the epidermis (Fig. 3 A ). The five species of Leandra have conical trichomes with rough surface and enations well developed or of reduced size (Tab. 3); in Leandra, glandular, dendritic and mixed trichomes were also registered (Tab. 3) (Fig. 3 B , 3C, 3D and 3E). Miconia leaves are devoid of conical trichomes, but commonly have dendritic trichomes (Fig. 3 F and 3G); glandular trichomes and vermiform trichomes were found only in Miconia fallax (Fig. 3 H ). It must be pointed out here that some authors (Baumgratz & Ferreira 1980Baumgratz JFA, Ferreira GL. 1980. Estudo da nervação e epiderme foliar das Melastomataceae do Município do Rio de Janeiro. Gênero Miconia. Seção Miconia. Rodriguésia 32: 161-169.; Oliveira 2007Oliveira JBS. 2007. Anatomia foliar como subsídio à taxonomia de Miconia Ruiz et Pav. (Melastomataceae) em Pernambuco-Brasil. MSc Thesis, Universidade Federal Rural de Pernambuco, Brazil.) have described the dendritic trichomes in M. dodecandra (Tab. 3) as stellate. The classification of dendritic trichomes compared to stellate trichomes is not very clear in the terminology of Wurdack (1986Wurdack JJ. 1986. Atlas of hairs for Neotropical Melastomataceae. Smithsonian Contributions to Botany 63: 1-80.). For this author, many of these stellate trichomes have an erect central arm, but show similarity to the dendritic type.

The taxonomic value of the trichome characters has been used in Melastomataceae by classic authors as Metcalfe & Chalk (1957Metcalfe CR, Chalk L. 1957. Anatomy of the dicotyledons - leaves, stem, and wood in relation to taxonomy with notes on economic uses. Oxford, Clarendon Press.), Baumgratz & Ferreira (1980Baumgratz JFA, Ferreira GL. 1980. Estudo da nervação e epiderme foliar das Melastomataceae do Município do Rio de Janeiro. Gênero Miconia. Seção Miconia. Rodriguésia 32: 161-169.) and Wurdack (1986Wurdack JJ. 1986. Atlas of hairs for Neotropical Melastomataceae. Smithsonian Contributions to Botany 63: 1-80.). Since then, several studies have been carried out as a taxonomic contribution to the family, such as Carmo et al. (2019Carmo A, Pacifico R, Gonçales-Silva R, Sbais PG, Fidanza K, Souza LA. 2019. Epidermal micromorphology and venation patterns of Microlicieae (Melastomataceae) leaves: Looking for new characters for the taxonomy of this neotropical tribe. Flora 261: 151-494.), Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.) and Gonçales et al. (2023Gonçales R, Carmo AAO, Santos AF, Souza LA. 2023. Floral indumentum of Miconia: In search of diagnostic characters for the tribe Miconieae. Revista Mexicana de Biodiversidad 94: e945272.). Our results confirm the diagnostic importance of the trichomes in Leandra and Miconia (Tab. 3), but that it must be associated with the structural characters of the leaf lamina. Conical trichomes and mixed trichomes (non-glandular and glandular) are present only in the five species of Leandra. Miconia trichomes are similar, in general they are dendritic with small variations in size and arms (terete or thin-walled arms). Leaves of M. valtherii and M. dodecandra are distinguished from the other investigated species, the former for exhibiting vermiform trichomes and the latter for presenting convex structures in high-domed (according to the terminology of Barthlott & Hunt 2000Barthlott W, Hunt D. 2000. Seed-diversity in the Cactaceae, subfamily Cactoideae. Succulent Plant Research 5. Authors edition.). It is emphasized that these domes resemble Wurdack’s (1986Wurdack JJ. 1986. Atlas of hairs for Neotropical Melastomataceae. Smithsonian Contributions to Botany 63: 1-80.) bulla-based trichomes without enations, except for the shape of the apex which is rounded rather than tapered.

Figure 3.
Morphology of trichomes in Scanning Electron Microscopy (SEM). A. Glabrous abaxial epidermal surface in Miconia pusilliflora. B. Long conical and filamentous trichomes, dendritic trichomes with short axis in Leandra melastomoides; detail of glandular trichome. C. Long conical trichomes with projections in Leandra microphylla. D. Conical and glandular trichomes in Leandra xanthocoma. E. Dendritic-type trichomes in Leandra purpurascens. F. Dendritic trichomes with short axis and laminar branches in Miconia splendens. G. Moderately long dendritic trichomes in the epidermis of Miconia valtherii. H. Vermiform trichomes in Miconia fallax. (ct=conical trichome; dt=dendritic trichome; gt=glandular trichome).

The conical trichomes on the leaves of Leandra species have raised doubts as to their essentially protodermal origin. Figure 2 A shows the base of some conical trichomes that can be interpreted as being of protodermal and subprotodermal origin. Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.) investigated conical trichomes from Leandra leaves and also questioned whether they are actually trichomes (protodermal origin) or emergences (protodermal and subprotodermal origin), based on Uphof (1962Uphof JCT. 1962. Plant hairs. In: Hummel K, Staeche K (eds.) Handbuch der Pflanzenanatomie. Berlin, Gebrüder Borntraeger. p. 205-206. ) and Evert (2013Evert RF. 2013. Anatomia das plantas de Esau: meristemas, células e tecidos do corpo da planta: sua estrutura, função e desenvolvimento. São Paulo, Blucher.). Our conception of a conical trichome for Leandra does not invalidate the thesis that this appendage may be an emergency, but as suggested by Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.) only its ontogeny can indicate its nature.

Some considerations about the structure of the leaf lamina that may be related to environmental factors

The leaves of the 15 species of Leandra and Miconia species investigated belong to plants collected in forests, but which can also occur in rupestrian fields (described on the labels of the material collected from the species). The leaf analysis has revealed structural characters that may have some relevant relationship with environmental factors, such as luminosity, water stress and wind. All species have stomata on the lower leaf surface and away from direct sunlight which can reduce water loss by transpiration, as suggested by Dickison (2000Dickison WC. 2000. Integrative plant anatomy. San Diego, Harcourt Academic Press.). Leaf structural variation affected by environmental factors has been reported in Leandra and Miconia species, such as M. ibaguensis (Bonpl.) Triana and M. stenostachya Schrank & Mart. ex DC. (Marques et al. 2000Marques AR, Garcia QS, Rezende JLP, Fernandes GW. 2000. Variations in leaf characteristics of two species of Miconia in the Brazilian cerrado under different light intensities. Tropical Ecology 41: 47-60.), M. tristis Spring and M. doriana Cogn. (Souza & Marquete 2000Souza RSOC, Marquete O. 2000. Miconia tristis Spring e Miconia doriana Cogn. (Melastomataceae): anatomia do eixo vegetativo e folhas. Rodriguésia 51: 133-142.), M. sellowiana (Boeger et al. 2008Boeger MRT, Gluzezak RM, Pil MW, Goldenberg R, Medri M. 2008. Variabilidade morfológica foliar de Miconia sellowiana (DC.) Naudin (Melastomataceae) em diferentes fitofisionomias no estado do Paraná. Revista Brasileira de Botânica 31: 443-452.), L. aurea (Cham.) Cogn., L. polystachya (Naudin) Cogn., L. sericea DC., M. albicans (Sw.) Triana, M. hyemalis A. St.-Hill. & Naudin and M. ligustroides (DC.) Naudin. (Gonçales-Silva et al. 2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.), and M. nervosa (Müller et al. 2020Müller AO, Franco AA, Ribeiro Júnior NG, Gressler E, Rocha VLP, Silva IVD. 2020. Estratégias adaptativas foliares de Miconia nervosa (Melastomataceae) na Amazônia Matogrossense. Rodriguésia 71: e01052018.). Other structural characters that can restrict water loss (sensuDickison 2000Dickison WC. 2000. Integrative plant anatomy. San Diego, Harcourt Academic Press.) were observed in these species, such as reduced leaf size, small undulations on the abaxial surface, thick cuticle on the adaxial surface, increase in trichome density, subepidermal layer (hypodermis), increase in the number of palisade parenchyma layers in the mesophyll, and presence of gelatinous fibers (Tab. 2 and 3).

Leandra microphylla (Tab. 2 and 3) may be considered a xeromorphic species, from the rupestrian fields of Paraná (Brazil); it exhibits leaf characters considered xeromorphic or sclerophyllous by Dickison (2000Dickison WC. 2000. Integrative plant anatomy. San Diego, Harcourt Academic Press.), such as reduced leaf size, undulations on the abaxial surface, trichome density, extensive development of palisade parenchyma in the mesophyll. The xeromorphic condition seems to also occur in L. nianga (Tab. 2 and 3) from Minas Gerais (Brazil), which shows some characters possibly related to water stress, such as undulations on the abaxial surface, three layers of palisade parenchyma, and gelatinous fibers.

Xeromorphic or sclerophyllous features have been recorded in other species (Tab. 2 and 3), such as thick cuticular layer, subepidermal crystals, hypodermis, isobilateral mesophyll and trichome density, but which may have a protective function under high light conditions (Dickison 2000Dickison WC. 2000. Integrative plant anatomy. San Diego, Harcourt Academic Press.), serving according to this author to increase the reflective power of the leaf and protect the photosynthetic cells of the mesophyll against excessive radiation. Xeromorphy was also found in some species of Leandra and Miconia by Gonçales-Silva et al. (2019Gonçales-Silva R, Almeida OJG, Souza LA. 2019. Caracteres estruturais foliares de Leandra e Miconia (Miconieae: Melastomataceae): importância taxonômica e ecológica. Boletim do Museu Paraense Emílio Goeldi-Ciências Naturais 14: 425-438.), but the authors reported that Leandra species have a smaller number of specialized structures for reduction or compensation of water loss than Miconia species.

Conclusion

The leaves of the 15 species of Leandra and Miconia provide many important structural features which have taxonomic value at the specific level. The analysis showed that some characters typically occur in some taxa: conical trichomes only in the genus Leandra and vermiform trichomes only in Miconia. Hypodermis in Miconia theaezans, quasi-concentric midrib in Leandra microphylla, and isobilateral mesophyll in Miconia sellowiana. Leaf features of potential taxonomic significance are the trichome types, presence of hypodermis, layer number of palisade parenchyma, occurrence of gelatinous fibers, midrib structure, and margin lamina structure. Xeromorphic and sclerophyllous features are present in Leandra and Miconia species, and Leandra microphylla may be considered a xerophyte.

Acknowledgements

The authors thank CAPES (Coordination of Higher Education Personnel Improvement, Brazil) and CNPq (National Council for Scientific and Technological Improvement, Brazil) for financing part of the work carried out (Master's and Research Productivity Scholarships), and to the Herbarium of the State University of Maringá (HUEM) and the Botanical Museum of Curitiba (MBM) for authorizing the collection of botanical material

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